Are the books written for blender 2.5 or 2.6 still useful for the latest blender 2.7 version?
Whether or not you can use information for a old version depends on what you are comparing: 


Cycles came out in 2.61, so anything prior will be for the blender internal.
The Python API has changed a fair bit since 2.5. So scripts, addons and python tutorials will most likely be incompatible. 
Many of the buttons may have sightly different places: example in 2.70 tabs for the toolbar came out.


In general anything written for 2.5 and up is still (more or less) relevant and use full now.  

If you get stuck, or something is not where it was then, ask a new question here.